1. Field of the Invention
The present invention relates to a method and apparatus for detecting the air permeability of vent holes formed in a belt-like sheet material and, more particularly, to an air permeability detecting method of comparing the permeating air resistance of vent holes set at a target air permeability with that of vent holes formed in the belt-like sheet material, and feeding back the difference as the comparison result to a laser unit, thereby controlling the intensity of a laser beam, and an apparatus for the same.
2. Description of the Related Art
Conventionally, as an apparatus for forming vent holes in a belt-like sheet material, a hole forming apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 63-188486 and the like are known. In this hole forming apparatus, a pulse laser beam is radiated to a mirror-surface pyramid, and a reflected laser beam is focused on a belt-like sheet material that moves in accordance with a predetermined radius of rotation, thereby forming fine holes in the belt-like sheet material.
As a method of detecting the porosity of the belt-like sheet material in which holes are formed by the above apparatus, a method of measuring the amount of light being transmitted through a hole portion and a method of measuring the amount of air permeating through the hole portion are known. Especially, as a method of measuring the porosity of chip paper used, for a example, in cigarettes, a method of measuring the permeating air amount is known.
Vent holes for diluting the inhalation concentrations of nicotine and tar are formed in the chip paper wound on the filter of a cigarette. The air permeability of the vent holes influences the taste of the cigarette. Therefore, it is important to control the air permeability at a predetermined value.
A conventional method of detecting the air permeability of the chip paper and an apparatus for the same will be described with reference to FIG. 1. As shown in FIG. 1, a pulse laser beam L oscillated by a laser unit 51 is reflected by a reflecting mirror 52 and focused by a condenser lens 53. The focused beam is radiated on chip paper C that moves at a predetermined speed. More specifically, the condenser lens 53 is adjusted such that the pulse laser beam L is focused on the chip paper C, thereby continuously forming holes in the moving chip paper C. The air permeability, as the porosity of the chip paper C, is determined in accordance with the conditions, e.g., the traveling speed of the chip paper C, the ON period, ON time, and intensity of the laser beam, and the material of the chip paper C.
As the chip paper C is moved, the vent holes formed in the chip paper C are moved at a predetermined speed in the longitudinal direction of the chip paper C. When the chip paper C passes above a dust collecting unit 54, dust from the chip paper C, generated during hole formation, is collected, and then the chip paper C passes above an air permeability detection unit 55. The air permeability detection unit 55 has a detection window 55a above which the chip paper C passes. A suction unit 57, for sucking the chip paper C on to the detection window 55a, is connected to the detection window 55a. An orifice 56 for converting an air flow rate to a pneumatic pressure is provided between the air permeability detection unit 55 and the suction unit 57. A differential pressure type converter 58 for converting a pressure difference between the pressure at the orifice 56 and the atmospheric pressure is provided between the air permeability detection unit 55 and the orifice 56.
The chip paper C is sucked, through the detection window 55a, by the suction pressure generated by the suction unit 57. At this time, the air permeability of the vent holes in the chip paper C, passing above the detection window 55a, is detected. More specifically, the flow rate of air drawn by the suction unit 57 is changed by a difference in air permeability. This flow rate is converted to a pneumatic pressure by the orifice 56. A change in pneumatic pressure is compared with the atmospheric pressure by the differential pressure type converter 58. The comparison result is converted to an electrical signal and detected as an air permeability.
The air permeability detected as the electrical signal in this manner is amplified by an amplifier 59 to a predetermined level and compared by a comparator circuit 60 with a preset target value, thereby obtaining a difference signal. The difference signal is supplied to an integration circuit 61 that stabilizes control, and is fed back as a control signal to the laser unit 51. As the permeating air amount of the vent holes of the chip paper C is changed, the oscillating laser intensity of the laser unit 51, to which the control signal is supplied, is changed.
The integration constant of the integration circuit 61 is set at an appropriate value considering the response time of the laser unit 51 and the traveling speed of the chip paper C.
In this manner, according to the conventional technique, variations in air permeability of the chip paper are detected as a change in pressure, a difference signal with respect to the atmospheric pressure as a reference is obtained, the difference signal is compared with a preset target value, and the comparison result is fed back to the laser unit, thereby controlling the laser oscillating intensity.
In this detecting method, however, the stability of the suction pressure at the suction unit for permeating air through the vent holes of the chip paper C is significant. More specifically, the suction pressure is changed depending on the material and air permeability of the chip paper C, variations in atmospheric pressure due to the weather and the like, the suction capability and stability of the suction fan, mechanical variations at other portions and the like. For example, when suction is performed at a pressure of 200 [mmAq], the suction pressure is changed by about 10 to 20 [mmAq] a day. Along with this change, the permeating air amount at the hole portion is also changed by about 3.4 [ml/mmAq] a day. When the suction pressure is changed by 10 [mmAq] a day, the permeating air amount is changed by about 34 [mml] a day. In this manner, if the suction pressure is not constant, it must be adjusted at least once a day.
When the suction pressure of the suction unit varies in this manner, the air permeability cannot be detected precisely, and the air permeability does not coincide with the target value. Even if the air permeability is stable, if the suction pressure varies due to the reasons described above, variations in suction pressure are detected as variations in air permeability. This detection signal is compared with the target value, and the comparison result is fed back to the laser unit. Therefore, the air permeability cannot be controlled correctly.
When the air permeability is not controlled correctly, the quality of product is degraded. Accordingly, an apparatus for always stabilizing the variations in suction pressure is required, not only increasing the entire size of the apparatus but also increasing the cost of the apparatus. In addition, it is very difficult to completely control the suction pressure to a predetermined value.